1. Field of the Invention
This invention relates in general to improvements in an automatic transmission, for example, for use in an automotive vehicle, and more particularly to an oil pump assembly of the automatic transmission including a boss securely receiving a stator shaft thereinside.
2. Description of the Prior Art
Automatic transmissions have been extensively used in automotive vehicles to automatically select speed changing stages by selectively hydraulically operating a variety of friction elements in connection with a transmission gear mechanism. Accordingly, an oil pump is essential for the automatic transmission in order to pressurize hydraulic oil serving as operating oil for the automatic transmission. The oil pump usually includes a pump element movably housed between a pump housing and a pump cover which are securely connected with each other. The oil pump is operatively interposed between a torque converter and the transmission gear mechanism and always driven by an engine. It is usual that a hollow stator shaft on which a stator of the torque converter is mounted is installed to the pump cover.
Now, if the pump cover of the large size is made of a heavy material such as cast iron regardless of unnecessity of supporting a large load. A weight increase is made in the automatic transmission. In this regard, it has been already put into practice that the pump cover is made of aluminum alloy. However, it is impossible that the stator shaft installed to the pump cover is made of aluminum alloy because the stator shaft receives reaction of the stator of the torque converter. Accordingly, the stator shaft is formed of a steel pipe and press-fitted to the pump cover so that they combine to form a one-piece structure.
In order to make efficient such press-fitting and to rotatably support the drum of a clutch of the transmission gear mechanism on the pump cover, the pump cover is integrally formed with a boss which projects axially in a direction opposite to the pump housing. The stator shaft is press-fitted into the boss from the direction of the pump housing.
However, difficulties have been encountered in such a pump cover arrangement, in which the rigidity of a part of the pump cover boss receiving the stator shaft is insufficient. In order to increase the rigidity, it is required to increase the thickness of the pump cover boss. This increases the diameter of component parts (such as a clutch drum) surrounding the boss, thereby making the outer diameter of the automatic transmission larger. Additionally, since the boss is integrally formed with the pump cover and made of aluminum alloy, the clutch drum rotatably supported on the pump cover boss wears the pump cover boss during a relatively short time period, thus shortening the life of the pump cover.